In fabricating products or structures comprising open celled or honeycomb ceramic components, it is sometimes necessary to utilize a ceramic cement to bond a plurality of such components together or to solid ceramic components, or to fill or plug selected cells (in whole or in part) within such components.
For joining lithium aluminosilicate ceramic components having low coefficients of thermal expansion, U.S. Pat. Nos. 3,189,512 and 3,634,111 disclose foaming ceramic cements comprising SiC foaming agent mixed lithium aluminosilicate ceramic material to form foamed ceramic cement bonds with and to fill the spaces between adjacent surfaces of those components. Thus, the components and the foamed cement have comparable properties (e.g. low coefficients of thermal expansion, chemical durability, etc.)
In recent years, occasions have arisen where it has been desirable to form the products or structures of cordierite ceramics also having low coefficients of thermal expansion because the cordierite ceramics provide properties (e.g. thermal and chemical) which are more beneficial than those of lithium aluminosilicate ceramics. Thus, for example, cordierite ceramic honeycomb structures have higher use (melting point) temperatures and are more resistant to sodium present in gas streams flowed through the cells of such structures (as in industrial heat recovery wheels). It is also desirable, then, to use foaming ceramic cements with these cordierite components that develop foamed cordierite cement bonds or plugs of comparable beneficial properties.
It is further desirable to form the cement mixture from unfired ceramic batch materials so as to avoid the expenditure of extra energy for prefiring or fusing such materials and from such materials as will form manganese cordierite (2MnO.multidot.2Al.sub.2 .multidot.5SiO.sub.2), which has a distinctly lower melting point than other types of cordierites but that is still suitably high for the desired uses. However, attempts to formulate foaming cements based on SiC foaming agent mixed with wholly raw (unfired) ceramic batch materials of combined composition designed to yield manganese cordierite resulted in foamed sintered clement masses that did not contain any significant codierite crystal phase. Rather, those resultant fired cements comprised mainly mullite and sometimes other noncordierite silicate phases lacking the desirable characteristics (e.g. durability) of cordierite. Similar undesirable results of failing to produce cordierite foamed cement were found with mixtures of silicon carbide with devitrifiable glass frit of the type shown in U.S. Pat. No. 4,191,583 to form Mg-Mn cordierite glass-ceramic when not mixed with SiC.